Sapogenins have achieved considerable usefulness as precursors for the preparation of steroid drugs, including sex hormones and cortisone. They are found in certain plant tissues in a combined glycosidal form known as saponins, and are generally removed therefrom and isolated by extraction.
Diosgenin is presently one of the principal sources of steroid drug "raw material" and is, perhaps, the sapogenin most widely used for this purpose. It occurs in glycosidal form in many species of the Liliaceae and Diosoreaceae families, but current commercial production of diosgenin relies primarily on two species of the Dioscoreaceae family Dioscorea mexicana (carbega de negra) and Dioscorea Composita (barbasco), in which the diosgenin is concentrated in the large, fleshy rhizomes, thus permitting yields up to approximately 1% of wet weight. These plants are not well suited for cultivation and require several years before they mature sufficiently to permit a reasonable harvest.
Removal and isolation of sapogenins from the glycosidal form found in certain plants has generally been accomplished by extraction of the fresh (wet) plant tissue with concentrated organic solvents such as benzene or combinations of benzene and concentrated alcohols, followed by acid hydrolysis, further extractions with benzene and in some cases chromatographic adsorption on alumina.
An equally suitable steroid precursor is solasodine which may be readily converted into 16-dehydroprognenolene acetate which is a very convenient percursor to steroid drugs. Solasodine occurs in glycosidal form in the fruit of huapag, a wood perennial of the genus Solanum, which is well suited for cultivation. This species thrives in certain high altitude regions in the tropic zone, for example, near Quito, Ecuador. Under such conditions this species will bear a small crop after two years, a much larger crop after five years, and will continue to bear all year round for many additional years.
The plant referred to herein as huapag is known by this name in the Quito area. In other parts of Ecuador, it is known as "jabon de pobres" and "huevo de tigre". It appears to be identical to one of the types of Solanum marginatum L., as described by Dunal (Dunal, M.F., Solanaceae, in de Candolle, Prodomus 13 (1): 1-690, 1852), the huapag being the type showing a spiny calyx.
Glycosides of solasodine are also known to occur in the fruits of several other species of the genus Solanum, particularly in tree Solanum. The plant referred to herein as tree solanum appears not to have been described in the botanical literature. It occurs wild near Archidona, Ecuador in the province of Napo (where it is known as apumpo) and also near Yacuambi, east of Loja, Ecuador, in the province of ZamoraChinchipe. A botanical definition of the tree Solanum follows:
Tree up to 20 meters or more with few to many scattered spines on trunk and main branches, young branches with stellate trichomes; leaves simple, from oval with nearly smooth margins to four or more lobes, nearly glabrous or with many stellate trichomes, blade usually unequal at base, about 3/4 as wide as long; flowers 6.0-7.0 cm. across, in clusters, purple fading to white with age, stellate hairs on both surfaces, calyx enlarging in fruit to 3.0-4.0 cm. across and becoming thickened (about 1.0 cm. thick around top of fruit); stamens yellow, tapered, another anther cm., filament 0.3 cm.; fruit single or in clusters (2-5), glabrous, green when mature, globose, 4.0-5.5 cm. in diameter, flesh greenishwhite; seeds numerous, flat, light-brown, about 0.3 cm. long.
Both tree Solanum and huapag are members of the stellate Leptostemonum group. The plants referred to herein as the stellate Leptostemonum group constitute the sub-genus Leptostemonum as defined by MacBride (J. F. MacBride, Flora of Peru, Field Museum of Natural History, Botanical Series, Volume 13, Part 5-B, Nov. 1, 1962) but further limited to exclude species which have only simple trichomes on the leaf petioles.
In glycosides, solasodine is chemically combined with sugars, and the identity and linkage of the sugar portion varies. For example, S. Marginatum is reported to contain solasonine and solamargine, both of which are glycosides of solasodine, but in which the sugar portions differ. In this application, the terms "glycosides" and "glycosides of solasodine" should be understood to mean any glycoside or mixture of glycosides of solasodine.
The difficulties of obtaining solasodine in usably pure form from these plant sources have seriously militated against the commercial development of this source of raw materials for hormones. It is known that glycosides of solasodine can be efficiently extracted from plant sources by extraction with aqueous lowboiling alcohols. However, extracts so obtained contain considerable proportions of other materials extracted from the plant sources along with the desired glycosides. While methods have been described for separating these impurities from the desired glycosides or from solasodine obtained from the glycosides by hydrolysis, such methods are laborious and result in loss of a considerable proportion of the desired steroids.
Furthermore, the hydrolysis step, especially if it be carried out with crude glycosides obtained in a practical extraction of said plants, normally leads to impure solasodine. This can be purified but the purification entails considerable effort and loss of product.
One problem in the hydrolysis of the glycosides of solasodine results from the tendency of these compounds to suffer dehydration to solasodiene under the vigorous conditions normally used for the hydrolysis. This tends to cause both a lowered yield of the desired aglycone and contamination of the solasodine with solasodiene.
Another problem in the hydrolysis of said glycosides to produce solasodine results from the tendency of the aglycone to be contaminated by impurities of unknown structure. One possible source of impurities of unknown structure is impurities in the glycosides used as starting material. All known convenient methods of obtaining said glycosides from natural sources yield products in which the glycoside content is much less than 100%. It is not known whether the impurities contained in glycosides obtained from natural sources are changed chemically in the course of acid hydrolysis used to convert the glycosides to the aglycone, but these impurities or their reaction products, or both, tend to appear as contaminants in the aglycone.
Another possible source of impurities of unknown structure is the decomposition of the glycosides or their hydrolysis products during the hydrolysis step. Besides producing by-products which contaminate the desired aglycone, these reactions cause a loss of yield.
Thus, a variety of impurities may be present in the solasodine resulting from the hydrolysis of crude glycosides thereof. Of all of these impurities, the most important in amount and in tendency to contaminate the product solasodine are those arising from impurities in the crude glycosides as these are normally extracted from plant material. This is true of the glycosides obtained according to the method given in this application, even though the glycosides so obtained are exceptionally pure as compared with those obtained by any convenient method previously disclosed.